(1) Field of the Invention
The present invention relates to a method of resetting an anode spike. More particularly, the present invention relates to a method of forming a satisfactory secondary anode in the spike pit formed after pulling out the spike in the case where the anode of a vertical type Soderberg electrolytic furnace for aluminum is consumed from its bottom thereby necessitating to reset the spike.
(2) Description of the Prior Art
As has been well-known, a vertical type Soderberg electrolytic furnace for aluminum is operated under the condition of keeping the molten electrolyte and the molten aluminum metal between a carbon cathode of the furnace body and a carbon anode (primary anode) suspended by spikes.
In a large electrolytic furnace, such carbon anode is suspended by as many as 50-60 spikes and is gradually comsumed from its bottom surface (for example by 14-15 mm per day) as the electrolysis proceeds. When the lower end of a spike comes to a predetermined height from the bottom surface of the carbon anode, the spike is pulled out and then set again at an adjusted height corresponding to the consuming rate of the anode. Since a pit is formed in the anode body after pulling out the spike, it is necessary to charge the pit with a packing paste previously prepared by compounding coke and pitch thus forming the secondary carbon anode, prior to the resetting of the spike.
Since the packing paste thus charged into the pit is subjected to rapid baking, the carbon anode thus formed (secondary anode) is porous and fragile, and has poor bondability with the primary anode. Thus, it may fall out of the primary anode during furnace operation thereby leading to anode troubles or increasing the contact resistance between the spike and the primary anode due to poor bondability. Furthermore, in order to avoid the undesirable effect caused by the formation of cavities after the falling out of the secondary carbon anode, the spike has to be maintained at a position considerably above the bottom surface of the primary anode, thereby causing an increase of the voltage drop through the anode.
Methods for overcoming such disadvantages are disclosed respectively in Japanese Patent Application Nos. 2724/1980 and 34682/1980.
Japanese Patent Application No. 2724/1980 proposes a secondary anode paste for use in a vertical type Soderberg electrolytic furnace for aluminum, in which the amount of finer particles compounded in the aggregate and the amount of a binder compounded with the aggregate have been increased respectively as compared to those in the primary anode paste. Specifically, while in the primary anode paste in general the finer particles which pass through Tyler standard mesh #200 occupy 30-40% by weight of the aggregate and the binder content is 25-35% by weight of the total amount of the paste, in the secondary anode paste the finer particles occupy 40-60% by weight of the aggregate and the binder content is 35-55%, preferably, 40-50% by weight of the total amount of the paste. It is also described therein that such secondary anode paste liquefies in a short time after being charged into the spike pits and then the liquefied paste rises up into any slight gap around the periphery of the spike to ensure sufficient electrical contact between the spike and the primary anode upon insertion of the spike, as well as forming a satisfactory secondary anode around and below the spike.
Japanese Patent Application No. 34682/1980 proposes a method of inserting a baked block of a configuration conforming to that of the top end of a spike into each of pits formed in the anode after pulling out the spike and, thereafter, setting the spike again. The baked block used in this method is prepared by starting from a secondary anode paste as the raw material containing an aggregate having a particle size distribution giving to the secondary anode substantially the same consuming property as that of the primary anode and a binder content of less than about 20% by weight which is lower than the content of about 30% by weight in the primary anode paste, molding the raw material into a desired configuration and, thereafter, baking it at 500.degree.-1100.degree. C. In the case where the anode has a non-flowable upper layer a method of using the baked block in combination with the secondary anode paste as described in Japanese Patent Application No. 2724/1980 is also proposed.
The present inventors have made studies for forming a secondary anode in the spike pits upon replacing the anode spike and have found that desirable effects comparable with those obtained by the method of using the baked block disclosed in Japanese Patent Application No. 34682/1980 can be attained by charging a molded packing paste into the spike pit and that the above-mentioned object can be attained much more economically, and have accomplished the present invention.